mod_sensor: Add initial UT for fwk handlers

This patch adds unit testing (UT) for the following
framework handlers:

sensor_init
sensor_dev_init
sensor_bind
sensor_start
sensor_process_bind_request
sensor_process_event

Change-Id: Idc11ab9eb32d5807d372ce0905643836dff3b2da
Signed-off-by: Andrew Graham <andrew.graham@arm.com>